Re: [PATCH 04/19] mm: page_alloc: Use jump labels to avoid checking number_of_cpusets

[Peter Zijlstra]

On Tue, May 13, 2014 at 10:45:35AM +0100, Mel Gorman wrote:
> +#ifdef HAVE_JUMP_LABEL
> +extern struct static_key cpusets_enabled_key;
> +static inline bool cpusets_enabled(void)
> +{
> +	return static_key_false(&cpusets_enabled_key);
> +}
> +
> +/* jump label reference count + the top-level cpuset */
> +#define number_of_cpusets (static_key_count(&cpusets_enabled_key) + 1)
> +
> +static inline void cpuset_inc(void)
> +{
> +	static_key_slow_inc(&cpusets_enabled_key);
> +}
> +
> +static inline void cpuset_dec(void)
> +{
> +	static_key_slow_dec(&cpusets_enabled_key);
> +}
> +
> +static inline void cpuset_init_count(void) { }
> +
> +#else
>  extern int number_of_cpusets;	/* How many cpusets are defined in system? */
>  
> +static inline bool cpusets_enabled(void)
> +{
> +	return number_of_cpusets > 1;
> +}
> +
> +static inline void cpuset_inc(void)
> +{
> +	number_of_cpusets++;
> +}
> +
> +static inline void cpuset_dec(void)
> +{
> +	number_of_cpusets--;
> +}
> +
> +static inline void cpuset_init_count(void)
> +{
> +	number_of_cpusets = 1;
> +}
> +#endif /* HAVE_JUMP_LABEL */

I'm still puzzled by the whole #else branch here, why not
unconditionally use the jump-label one? Without HAVE_JUMP_LABEL we'll
revert to a simple atomic_t counter, which should be perfectly fine, no?

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